Selective laser sintering (SLS) is an important branch of additive manufacturing techniques, in which, on the basis of concepts of dispersing and accumulated forming, a solid model is formed by selectively scanning and sintering powdery material by laser, laminated manufacturing and superposition layer by layer. SLS is different from the conventional processes such as dislodge forming, splice forming and forced forming, where models and functional parts of high polymer material, ceramic material, metal material and various composites are processed by accumulation of material. Independent of the complexity of formed parts, SLS has been widely applied in the biomedical field. By SLS, personalized and complex medical products can be produced most efficiently to maximally match the needs of patients.
As a wholly aromatic semi-crystalline polymer, PEEK has the advantages of good mechanical performance and excellent radiation resistance and insulation. In many special fields, PEEK can be used to replace conventional material, for example, metal material and ceramic material. With the use of PEEK, the weight is reduced without influencing the performance in use. In the field of medical instruments, PEEK is increasingly applied in clinical practices of medical products for spinal surgeries, injuries and orthopedic treatments. Compared with other implants, PEEK has good biocompatibility and adjustable mechanical property, and is invisible in CT and MRI. However, the application of PEEK in the chemical field is restricted due to the lack of bioactivity. As a main component of bones and teeth of vertebrates, the content of HA is 96 wt % in human tooth enamel and 69 wt.% in human bones. Compared with the conventional bone-substitute material, for example, stainless steel, titanium alloy and ceramic material, HA has excellent osteoinduction and is thus highly conducive for the adhesion and growth of bone cells on its surface. Therefore, HA has been widely applied in bone tissue engineering. By the release of calcium ions and phosphate ions, closed bonding can be formed between the implanted HA and tissues, with new tissues gown there. In this way, the implant is bonded with the bones. HA will be more bioactive at a finer grain size. The combination of NANO-HA and PEEK can significantly improve the bioactivity of PEEK while ensuring enough intensity, and thus has great value for clinic use and broad market prospect.
At present, there are main four methods for preparing powdery composites for SLS, i.e., mechanical mixing, twin-screw extrusion and freeze grinding, solvent mixing and solvent precipitation. However, all the four preparation methods have obvious disadvantages in the preparation of powdery PEEK/NANO-HA composites for SLS, specifically:
(1) Mechanical mixing means that two kinds of powder are mixed by a mechanical mixing device. This method is simple in operation. However, by this method, it is unable to uniformly disperse nano-fillers in the high polymer substrate at a nanometer level.
(2) Twin-screw extrusion and freeze grinding means that two kinds of material are molten, blended, extruded and granulated by a twin-screw extruder, and then by freeze grinded to prepare powder for SLS. By this method, two kinds of material can be uniformly mixed. However, since PEEK has high intensity and toughness, it is unable to produce PEEK into powder, having a small particle size of 10-100 μm, for SLS by freeze grinding, and the powdery particles are highly irregular in shape with poor fluidity.
(3) Solvent mixing usually requires some organic solvents which are harmful to the environment and human beings. The residual solvents will have adverse effect on the biocompatibility of the composites. In addition, the dispersion of NANO-HA in the organic solvents is usually realized in aid of surfactants. The residual surfactants will degrade the biocompatibility of the composites. For example, N,N′-dimethylformamide (solvent) was used in the preparation of PEEK/NANO-HA composites for SLS, by Fuhua, et al.
(4) Solvent precipitation is generally applicable to the preparation of powdery nylon composites. For example, in patent ZL200710053668.2, inorganic nano-filler reinforced nylon composites for SLS were prepared by solvent precipitation; and in patent ZL200710053667.8, clay-reinforced nylon composites for SLS were prepared by solvent precipitation. Since PEEK is quite resistant to solvents, this method is not applicable to the preparation of powdery PEEK composites.